In a screw extruder, material, usually some form of plastic, is forced under pressure to flow through an contoured orifice in order to shape the material. Screw extruders are generally composed of a housing, which is usually a cylindrical barrel section, surrounding a central motor-driven screw. At a first end of the barrel is a feed housing containing a feed opening through which new material, usually plastic particles, is introduced into the barrel. The material is then conveyed by the screw toward the second end of the barrel through a melting zone where it is heated under carefully controlled conditions to melt the material. It is then conveyed through a melt-conveying zone, also called a pumping zone because it acts as a pump for the melted material. The melted plastic is finally pressed through a shaped opening or die to form the extrudate.
A common problem in this process is fluctuations due to variations in feeding and conveying of the plastic particles in the barrel of the extruder. The driving force for the forward conveyance of the material is the frictional force between the plastic particles and the barrel surface in the feed section. Plastics such as polypropylene have low coefficients of friction against steel. With a smooth barrel surface, the frictional force can be too low to ensure consistent forward conveyance, resulting in erratic performance and poor efficiency of material conveying.
A common solution to feeding problems due to insufficient barrel friction is to machine grooves into the throat of the barrel in the feed section of the extruder. This has the advantage of providing high extruder output with more stable extruder performance. Additionally, it may provide the ability to process plastics with very high molecular weights.
The disadvantages of using grooves include the generation of high pressures, which can lead to high motor loads, high melt temperatures and rapid wear of the screw and barrel. Permanent grooves also require a higher start-up torque than a smooth barrel, necessitating a more powerful electric motor for start-up than will be required in subsequent operation, as well as stronger screw and barrel parts.
It is important that the feed section be thermally insulated from the melting section. Due to heat generated by friction, the feed section may additionally require cooling to make sure the plastic particles do not melt in the grooved section of the extruder. Thus, it may be necessary to provide cooling systems for barrels with grooved throats. This provides yet another disadvantage because a great amount of energy is lost through the intensive cooling of the grooved barrel section. Experimental studies have shown that as much as 30-40% of the mechanical energy may be lost through the cooling water.
The form and size of the pellets also affect the frictional characteristics of the material. For instance, in a grooved throat designed to operate with essentially round pellets, cubic pellets may be conveyed with a much higher efficiency. This may require that barrels with non-adjustable grooves be designed for specific materials.
Another disadvantage of grooved barrels is that material can accumulate in the grooves. This can cause problems when changing from one product to another as mixing in this residue can affect the purity of the subsequent material.
Poor performance and rapid wear of the equipment may also result when there is a mismatch of efficiencies between the screw in conveying the material and the heating zones in melting the material. If the screw conveys the solid material too efficiently, it may deliver it faster than the melting and melt conveying zones can process the material. This can result in erratic operation as the machine drives unmelted and unplasticated material through the melting zone. This may, in extreme cases, result in very high pressures in the barrel that can lead to risk of barrel rupture. It is thus advantageous to have a method of controlling the conveying efficiency in the feed zone so that it may be matched with that of the melting and melt-conveying zones.
The efficiency of conveying of material by the screw may be controlled by several factors. One way to adjust this conveying efficiency is to change the temperature of the barrel in the feed section. This is often done, but has the disadvantage that temperature adjustment has only a weak effect on conveying efficiency. Similarly, the temperature of the screw in the feed section may be adjusted. However, this method also has a weak effect and is additionally more complicated to control than barrel temperature.
As discussed above, the amount of frictional force at the barrel is a factor in the conveying efficiency. The presence and the specific geometry of grooves in the barrel can be used to directly change this efficiency. The number of grooves, the length of grooves, and the groove depth determine the frictional effect. A continuous adjustment of the number of grooves is not likely. Adjustment of the length of the grooves is possible, but generally will be mechanically complex due to the large change in groove length that is likely to be required. The typical axial length of the grooves is two to five barrel diameters. Thus the adjustment length would have to be in the range of two to five barrel diameters as well. This may be an impractical length to adjust. Therefore, the most convenient method of controlling the frictional effect and thus the conveying efficiency would appear to be to adjust the depth of the grooves. The groove depth is typically from 0-0.12 inches (0-3 mm). The range of adjustment would thus be only about 0.12 inches, which is typically about half the size of the plastic pellets usually used. This method has the advantage of being immediately effective with no time lag involved, in contrast with temperature control methods.
If the groove depth is adjustable to zero, the difficulties previously mentioned concerning start-up torque and necessary over-design of the motor may be avoided. This results in cost savings since the motor, screw and barrel parts may be designed for smaller stresses. Once operating speed has been achieved, the grooves may be continuously adjusted to obtain optimal conveying efficiency.
U.S. Pat. No. 4,678,339 by Peiffer et al. describes a mechanism for adjusting the depth of grooves in the feed housing of a screw extruder. Adapters in grooves in the inner surface of the barrel are biased radially outward by helical springs surrounding the adapter's attachment bolts that are grouped in circumferential sets. The heads of these bolts are attached to rollers that contact the inner surface of a cam ring. Rotation of the cam ring relative to the bolt heads allow the bolts to be moved radially inward or outward, and thus to raise or lower the adapters in the grooves. Rotation of the ring does not allow for movement of individual bolts in a circumferential set, as all move in unison. As a consequence, the adapters are not capable of such fine adjustment as would a screw extruder with individually adjustable adapters and therefore operating parameters and objectives may not be as precisely controlled.
The cam ring mechanism of Peiffer '339 is comparatively bulky and complex. It can also interfere with incorporation of a cooling system into the feed zone of the barrel. As discussed earlier, good cooling is important to avoid melting of the plastic, which will have an effect on frictional properties and conveying efficiency.
For the foregoing reasons, there is a need for a screw extruder with individually adjustable groove depths and whose adjustment mechanisms are less complex, bulky and expensive to use and maintain.